Halogen-bonded Complexes Research Articles

The 15N NMR chemical shift in the characterization of weak halogen bonding in solution.

We have studied the applicability of 15N NMR spectroscopy in the characterization of the very weak halogen bonds of nonfluorinated halogen bond donors with a nitrogenous Lewis base in solution. The ability of the technique to detect the relative strength of iodine-, bromine- and chlorine-centered halogen bonds, as well as solvent and substituent effects was evaluated. Whereas computations on the DFT level indicate that 15N NMR chemical shifts reflect the diamagnetic deshielding associated with the formation of a weak halogen bond, the experimentally observed chemical shift differences were on the edge of detectability due to the low molar fraction of halogen-bonded complexes in solution. The formation of the analogous yet stronger hydrogen bond of phenols have induced approximately ten times larger chemical shift changes, and could be detected and correlated to the electronic properties of substituents of the hydrogen bond donors. Overall, 15N NMR is shown to be a suitable tool for the characterization of comparably strong secondary interactions in solution, but not sufficiently accurate for the detection of the formation of thermodynamically labile, weak halogen bonded complexes.

Photoresponsive ionic liquid crystals assembled via halogen bond: en route towards light-controllable ion transporters.

We demonstrate that halogen bonding (XB) can offer a novel approach for the construction of photoresponsive ionic liquid crystals. In particular, we assembled two new supramolecular complexes based on 1-ethyl-3-methylimidazolium iodides and azobenzene derivatives containing an iodotetrafluoro-benzene ring as XB donor, where the iodide anion acted as an XB acceptor. DSC and X-ray diffraction analyses revealed that the preferred stoichiometry between the XB donors and acceptors is 2 : 1, and that the iodide anions act as bidentate XB-acceptors, binding two azobenzene derivatives. Due to the high directionality of the XB, calamitic superanions are obtained, while the segregation occurring between the charged and uncharged parts of the molecules gives rise to a layered structure in the crystal lattice. Despite the fact that the starting materials are non-mesomorphic, the halogen-bonded supramolecular complexes exhibited monotropic lamellar liquid-crystalline phases over broad temperature ranges, as confirmed with polarized optical microscopy. Due to the presence of the azobenzene moieties, the LCs were photoresponsive, and a LC-to-isotropic phase transition could be obtained by irradiation with UV light. We envisage that the light-induced phase transition, in combination with the ionic nature of the LC, provides a route towards light-induced control over ion transport and conductance in these supramolecular complexes.

First structural evidence of a Se–Br–Br halogen-bonded molecular complex

Bromination of the bicyclic selenoether (2,6-dibromo-9-selenabicyclo[3.3.1]-nonane) affords the products ranging from the unique molecular complex C8H12Br2Se–Br2 to the ionic polybromide complex [C8H12Br2SeBr+]Br5.

Endo-/exo- and halogen-bonded complexes of conformationally rigid C-ethyl-2-bromoresorcinarene and aromatic N-oxides

Unlike that of the conformationally flexibleC-ethyl-2-methylresorcinarene the structurally more rigidC4vcavity inC-ethyl-2-bromoresorcinarene prefers only small aromaticN-oxides stabilized through C–H⋯π interactions.

Quantitative Assessment of Halogen Bonding Utilizing Vibrational Spectroscopy.

A total of 202 halogen-bonded complexes have been studied using a dual-level approach: ωB97XD/aug-cc-pVTZ was used to determine geometries, natural bond order charges, charge transfer, dipole moments, electron and energy density distributions, vibrational frequencies, local stretching force constants, and relative bond strength orders n. The accuracy of these calculations was checked for a subset of complexes at the CCSD(T)/aug-cc-pVTZ level of theory. Apart from this, all binding energies were verified at the CCSD(T) level. A total of 10 different electronic effects have been identified that contribute to halogen bonding and explain the variation in its intrinsic strength. Strong halogen bonds are found for systems with three-center-four-electron (3c-4e) bonding such as chlorine donors in interaction with substituted phosphines. If halogen bonding is supported by hydrogen bonding, genuine 3c-4e bonding can be realized. Perfluorinated diiodobenzenes form relatively strong halogen bonds with alkylamines as they gain stability due to increased electrostatic interactions.

Multifrequency Multiresonance EPR Investigation of Halogen-bonded Complexes Involving Neutral Nitroxide Radicals

Abstract Halogen-bonded complexes with neutral nitroxide radicals as the Lewis base have been investigated in liquid and frozen solutions by multifrequency CW and pulse EPR spectroscopies, including ENDOR and ELDOR-detected NMR (EDNMR) techniques. The non-covalent interaction with iodopentafluorobenzene as halogen-bond donor is shown to affect a variety of EPR parameters of the stable nitroxide radicals. In liquid solution, only bulk effects on the EPR signal, i.e. isotropic g value, isotropic 14N hyperfine coupling and linewidth, are observed. Experiments on frozen solutions allow for a more in-depth dissection of complexing effects. W-band EPR spectra at cryogenic temperatures exhibit multiple signal components of different 14N hyperfine interactions and spectral widths. This demonstrates the coexistence of several halogen-bonded complexes that differ in donor-acceptor binding geometries. These complexes have different relaxation properties, which allow for their spectral discrimination. 19F ENDOR experiments prove the origin of these effects to be different specific intermolecular interactions rather than a consequence of changes in the solvation environment. The EPR spectra yet reveal a strong influence of solvent composition on the amount of the various complexes formed. The introduced methodology for the characterization of such adducts improves our understanding of halogen bonding and could be helpful in the development of tailor-made donors and complexes for specific applications.

Theoretical investigation of molecular interactions between sulfur ylide and hypohalous acids (HOX, X═F, Cl, Br, and I)

ABSTRACTIn the present study, the complexes of sulfur ylide (SY) with hypohalous acids (HOX; X═F, Cl, Br, and I) were theoretically studied using the MP2/aug-cc-pVTZ computational level. Four types of structures were obtained for these complexes including two kinds of hydrogen bonds (HBs) (SY: HOX─I and SY: HOX─II) and two kinds of halogen bonds (SY: XOH─I and SY: XOH─II). According to the energetic results, the SY:XOH─I complexes are stronger for X═Br and I, while the SY:HOX─I complexes are favorable for X═Cl and Br; also, the SY:HOX─II complexes are stronger than SY:XOH─II except for SY:IOH─II, which is stronger than its counterpart (SY:HOI─II). Moreover, on comparing SY:HOX─I and II HBs complexes, the former is more stable than the latter, while between the SY:XOH─I and II halogen bond complexes (X═Cl, Br, and I), the SY:XOH─I ones are more stable. The strength, properties, and nature of interactions were also analyzed using the natural bond orbital and atoms in molecules theories.

Nitroxide Radical Spin Probes for Exploring Halogen‐Bonding Interactions in Solution

The synthesis of 2,3,5,6-tetrafluoro-4-iodobenzyl tert-butyl nitroxide (2-I) and its use as spin probe for the detection of halogen-bond (XB) complexes by EPR is reported. Formation of a XB complex between 2-I and several XB acceptors was evidenced by a significant change in the value of the benzylic hyperfine splitting upon complexation. Thermodynamic parameters for the formation of XB complex with quinuclidine were obtained by recording EPR spectra in the temperature range 203-294 K. In addition, competitive experiments allowed for the measurement of the equilibrium constant of the XB complex with a chloride anion. The proposed procedure constitutes the first direct EPR methodology providing a reliable determination of the strength of the XB bond in solution.

Boron as an Electron-Pair Donor for B⋅⋅⋅Cl Halogen Bonds.

MP2/aug'-cc-pVTZ calculations were performed to investigate boron as an electron-pair donor in halogen-bonded complexes (CO)2 (HB):ClX and (N2 )2 (HB):ClX, for X=F, Cl, OH, NC, CN, CCH, CH3 , and H. Equilibrium halogen-bonded complexes with boron as the electron-pair donor are found on all of the potential surfaces, except for (CO)2 (HB):ClCH3 and (N2 )2 (HB):ClF. The majority of these complexes are stabilized by traditional halogen bonds, except for (CO)2 (HB):ClF, (CO)2 (HB):ClCl, (N2 )2 (HB):ClCl, and (N2 )2 (HB):ClOH, which are stabilized by chlorine-shared halogen bonds. These complexes have increased binding energies and shorter B-Cl distances. Charge transfer stabilizes all complexes and occurs from the B lone pair to the σ* Cl-A orbital of ClX, in which A is the atom of X directly bonded to Cl. A second reduced charge-transfer interaction occurs in (CO)2 (HB):ClX complexes from the Cl lone pair to the π* C≡O orbitals. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants, 1x J(B-Cl), across the halogen bonds are also indicative of the changing nature of this bond. 1x J(B-Cl) values for both series of complexes are positive at long distances, increase as the distance decreases, and then decrease as the halogen bonds change from traditional to chlorine-shared bonds, and begin to approach the values for the covalent bonds in the corresponding ions [(CO)2 (HB)-Cl]+ and [(N2 )2 (HB)-Cl]+ . Changes in 11 B chemical shieldings upon complexation correlate with changes in the charges on B.

B4H4 and B4(CH3)4 as Unique Electron Donors in Hydrogen-Bonded and Halogen-Bonded Complexes.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on B4H4 and B4(CH3)4 to investigate the base properties of these molecules with Td symmetry. Each face of the tetrahedral structure of B4H4 and B4(CH3)4 is stabilized by a two-electron, three-center B-B-B bond. The face uses these two electrons to act uniquely as an electron-pair donor for the formation of stable hydrogen-bonded and halogen-bonded complexes with C3v symmetry. The hydrogen-bonded complexes are B4H4:HY and B4(CH3)4:HY, with HY = HNC, HF, HCl, HCN, and HCCH; the halogen-bonded complexes are B4H4:ClY and B4(CH3)4:ClY, with ClY = ClF, ClCl, ClNC, ClCN, ClCCH, and ClH. The absolute values of the binding energies of the hydrogen-bonded complexes B4(CH3)4:HY and of the halogen-bonded complexes B4(CH3)4:ClY are significantly greater than the binding energies of the corresponding complexes with B4H4. The binding energies of each series correlate with the distance from the hydrogen-bonded H atom or halogen-bonded Cl atom to the centroid of the interacting face. Charge transfer stabilizes all complexes and occurs from the B2-B3-B4 orbital of the face to the antibonding H-X orbital of HY in hydrogen-bonded complexes and to the antibonding Cl-X orbital of ClY in halogen-bonded complexes, with X being the atom of Y that is directly bonded to either H or Cl. For fixed HY, EOM-CCSD spin-spin coupling constants J(X-B1) are greater than J(X-Bn) for complexes B4H4:HY, even though the X-B1 distances are longer. B1 and Bn are the atoms at the apex and in the interacting face, respectively. Similarly, for complexes B4H4:ClY, J(Cl-B1) is greater than J(Cl-Bn). In the halogen-bonded complexes, both coupling constants correlate with the corresponding distances.

A DFT study of Lp···π/halogen bond competition in complexes of perhalogenated alkenes with oxygen/nitrogen containing simple molecules

The possible noncovalent lone pair‐π/halogen bond (lp···π/HaB) complexes of perhalogenated unsaturated C2ClnF4−n (n = 0–4) molecules with four simple molecules containing oxygen or nitrogen as electron donor, formaldehyde (H2CO), dimethyl ether (DME), NH3, and trimethylamine (TMA), have been systematically examined at the M062X/aug‐cc‐pVTZ level. Natural bond orbital (NBO) analysis at the same level is used for understanding the electron density distributions of these complexes. The progressive introduction of Cl atom on C2ClnF4−n influences more on the lp···π complexes over the corresponding HaB ones. Within the scope of this study, gem‐C2Cl2F2 is the best partner molecule for lp···π interaction with the simple molecules, coupled with the greatest interaction energy (IE) and second‐order orbital interaction [E(2) value], whereas C2F4 is the poorest one. The C2Cl3F·H2CO and C2Cl4·H2CO complexes exhibit reverse lp···π bonding, while the Z/E‐C2Cl2F2·NH3, C2Cl3F·NH3 and C2Cl4·NH3 complexes perform half‐lp···π bonding according to the NBO analysis. The lp···π interaction involving the oxygen/nitrogen and the π‐hole of C2ClnF4−n overwhelms the HaB involving the oxygen/nitrogen and the σ‐hole of the Cl atom. The electron‐donating methyl groups contribute significantly to the two competitive interactions, therefore, DME and TMA engage stronger in the partner molecules than H2CO and NH3. Our theoretical study would be useful for future experimental investigation on noncovalent complexes. © 2016 Wiley Periodicals, Inc.

Anionic complexes of F− and Cl− with substituted methanes: Hydrogen, halogen, and tetrel bonds

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the anionic complexes X−:CX(FnH3−n), for X=F, Cl, and n=0–3. These complexes are stabilized by tetrel, hydrogen, and halogen bonds. Hydrogen-bonded complexes are the most stable complexes and halogen-bonded complexes are the least stable, with one exception. Charge-transfer across intermolecular bonds stabilizes all complexes, and occurs from the anion lone-pair to a σ∗ orbital of the substituted methane. EOM-CCSD spin–spin coupling constants 1tJ(X–C) across intermolecular tetrel bonds, 2hJ(C–X) across hydrogen bonds, and 1xJ(Cl–Cl) and 2xJ(C–Cl) across halogen bonds correlate with intermolecular distances.

The mutual influence between π-hole pnicogen bonds and σ-hole halogen bonds in complexes of PO2Cl and XCN/C6H6 (X = F, Cl, Br)

The bimolecular and termolecular complexes involving PO2Cl and XCN/C6H6 (X = F, Cl, Br) were designed to form the π-hole pnicogen bonds and σ-hole halogen bonds, to compare the two types of interactions and investigate the mutual influences between them. PO2Cl was used as simultaneous π-hole and σ-hole donor; it can interact with electron donor to form π-hole pnicogen bond and σ-hole halogen bond. The π-hole interactions are stronger than the σ-hole interactions, in both the bimolecular and the termolecular complexes. Comparing the mutual effects of the π-hole interactions and σ-hole interactions, the π-hole interaction has a greater influence on the σ-hole interaction than vice versa. With the addition of σ-hole halogen bond, the V S,max value outside the π-hole region of PO2Cl becomes decreasingly positive, resulting in a weaker π-hole interaction. With the addition of π-hole pnicogen bond, the V S,max value outside the σ-hole region of PO2Cl becomes small, also resulting in a weaker σ-hole interaction. The π-hole pnicogen bond and σ-hole halogen bond weaken each other, i.e., there is a negative cooperative effect in the termolecular complexes.

2,2′:6′,2″-Terpyridine Trimethylplatinum(IV) Iodide Complexes as Bifunctional Halogen Bond Acceptors

Three new organometallic trimethylplatinum(IV) iodide complexes of 2,2′:6′,2″-terpyridines have been synthesized and characterized by 1H NMR spectroscopy, mass spectrometry, elemental analysis, and single crystal X-ray diffraction analysis. The X-ray crystal structures of PtMe3I(L) complexes 1–3 {L for 1 = 4′-chloro-2,2′:6′,2″-terpyridine, 2 = 4′-(4-cyanophenyl)-2,2′:6′,2″-terpyridine, and 3 = 4′-(4-tolyl)-2,2′:6′,2″-terpyridine} reveal distorted octahedral coordination geometry of the platinum(IV) metal centers with bidentate coordination of the terpyridine ligands. Complexation of 1–3 with iodopentafluorobenzene (IPFB) afforded single-crystal structures of halogen bond (XB) complexes 1a–3a, in which the Pt-complexes act as bifunctional XB bond acceptor wherein the pendant pyridine nitrogen atom and the platinum bound iodine atom act as XB acceptors toward the polarized iodine of IPFB. In addition, complexation of 1 with 1,4-diiodotetrafluobenzene (DITFB) afforded single-crystal structure of dimeric XB c...

A theoretical simulation of the resonant Raman spectroscopy of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes.

The resonant Raman spectra of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes have been studied in the framework of a 2-dimensional model previously used in the simulation of their UV-visible absorption spectra using time-dependent techniques. In addition to the vibrational progression along the dihalogen mode, a progression is observed along the intermolecular mode and its combination with the intramolecular one. The relative intensity of the inter to intramolecular vibrational progressions is about 15% for H2O⋯Cl2 and 33% for H2O⋯Br2. These results make resonant Raman spectra a potential tool for detecting the presence of halogen bonded complexes in condensed phase media such as clathrates and ice.

Comparison of π-hole tetrel bonding with σ-hole halogen bonds in complexes of XCN (X = F, Cl, Br, I) and NH3.

In addition to the standard halogen bond formed when NH3 approaches XCN (X = F, Cl, Br, I) along its molecular axis, a perpendicular approach is also possible, toward a π-hole that is present above the X-C bond. MP2/aug-cc-pVDZ calculations indicate the latter geometry is favored for X = F, and the σ-hole structure is preferred for the heavier halogens. The π-hole structure is stabilized by charge transfer from the NH3 lone pair into the π*(CN) antibonding orbital, and is characterized by a bond path from the N of NH3 to the C atom of XCN, a form of tetrel bond. The most stable 2 : 1 NH3/XCN heterotrimer for X = F and Cl is cyclic and contains a tetrel bond augmented by a pair of NHN H-bonds. For X = Br and I, the favored trimer is noncyclic, stabilized by a tetrel and a halogen bond.

Comparison of tetrel bonds and halogen bonds in complexes of DMSO with ZF3X (Z = C and Si; X = halogen)

A theoretical study of the complexes formed by dimethylsulfoxide (DMSO) with ZF3X (Z = C and Si; X = halogen) has been performed at the MP2/aug-cc-pVTZ level.

Extending the halogen-bonded supramolecular synthon concept to 1,3,4-oxadiazole derivatives

A series of five crystal structures of 1:1 halogen-bonded complexes were obtained from 4-[5-(4-alkoxyphenyl)-1,3,4-oxadiazole-2-yl]pyridine and 1,3,5-trifluorotriiodobenzene. Electronic structure calculations show that the N(oxadiazole)⋯I interaction in the new synthon is as strong as the classic N(pyridine)⋯I interaction. Oxygen to sulfur atom subsitution on the oxadiazole ring results in a different supramolecular packing where the N(pyridine)⋯I interaction is favored, which could be rationalized by the changes in the molecular electrostatic potential predicted from the theoretical calculations.

Halogen transfer through halogen bonds in halogen-bound ammonia homodimers.

Ab initio MP2(full)/aug-cc-pVTZ calculations have been carried out to investigate the halogen transfer between haloamines and ammonia. The results show that the formation of a halogen bond complex between ammonia and the protonated N-haloamine is a preliminary step in the halogen transfer process. The complexation energies, optimized geometries, topology of electron density and potential energy surfaces for halogen transfer in these complexes have been analysed. It has been found that halogen-bound ammonia homodimers ([H3NXNH3]+, with X = Cl, Br, I) formed by interaction between NH3 and H3NX+ are symmetric, and energetically more stable than the corresponding complexes formed between NH3 and H2NX. Calculated potential energy surfaces for the transfer of the central halogen atom between the two NH3 units in [H3NXNH3]+ show single well and double well potentials for short and large N-N distances, respectively. The particular case of fluorine complexes has also been analysed. The results provide an explanation for some of the experimental facts observed for halogen transfer reactions between amines in aqueous solution.

Effects of substituents and charge on the RCHO⋯X–Y {X = Cl, Br, I; Y = –CF3, –CF2H, –CFH2, –CN, –CCH, –CCCN; R = –OH, –OCH3, –NH2, –O−} halogen-bonded complexes

Noncovalent interactions involving halogen bonding interactions, one of the emerging interactions due to its directionality, have been a subject of interest for various researchers, owing to its role played in construction of supramolecular structures and crystal engineering. In this article, the RCHO⋯X–Y {X = Cl, Br, I; Y = –CF3, –CF2H, –CFH2, –CN, –CCH, –CCCN; R = –OH, –OCH3, –NH2, –O−} halogen-bonded complexes have been investigated with aid of quantum chemical calculations at MP2/aug-cc-pVDZ(-PP) level. The geometrical, spectroscopic and energetic properties have been analyzed for these complexes with QTAIM, MEP and NBO analyses. For same RCHO molecule, the interaction energies increase in the order of O⋯Cl < O⋯Br < O⋯I and for the same X–Y; the trend is (HO)CHO < (CH3O)CHO < (H2N)CHO. The present article also compares the neutral complexes involving (HO)CHO, (CH3O)CHO and (H2N)CHO carbonyl molecules with those of charged carbonyl species HCOO−. It has been found that strength of the latter complexes is highest among all of the four. The role of substituents in the complex formation has been analyzed on the basis of results obtained from MEP, QTAIM and NBO studies.